Hello K. O.,
your first step is to pick an organization through which to become certified, and you will have many to choose from. Here on this website are many organizations listed by category where you have a good comparison. Here is the link http://www.ideafit.com/fitness-certifications. If you have a specific place in mind where you think of teaching, I would find out whether they have a preference for one certification over another. Not all certifications are created equal, and some carry greater standing in the industry than others.
IF your intention is to teach group fitness, I would start with a general group fitness certification like the one from ACE before moving into a specialty. However, there are organizations that specialize such as the Senior Fitness Association (SFA), the Aquatic Exercise Association (AEA) or Yoga Alliance (and there are plenty more).
Some certifications require that you attend a workshop, others (such as ACE) can be done entirely at home and online with a test at a proctored site. This may also be a factor in which certification you choose because travel expenses can make a certification significantly more expensive.
Hope this helps.
Here are a few good articles from IDEA on that subject:
I would just add that in yoga what is considered ‘certified’ follows a somewhat different model than in the other areas you mention. It used to be much more fluid, but in the past couple of decades it has become usual for people to take a training that covers specified basic material.
Yoga, unless you are teaching a form of fitness that has aspects of yoga, rather than yoga typically you want to get yourself on the yoga alliance registry. The alliance does not train and certify itself, but simply maintains standards and provides a way to compare across trainings. Anyone can pay a fee and become a registered yoga teacher, but to begin to teach typically one takes either a 200 or 500 hour training. T
If you want to teach asanas, or add elements of yoga to your regular classes then just start with your national certification.
Typically a national group exercise certification is the way to start for most fitness instructors, and some also take a degree, though a good national certification is generally what is required in most places to teach.
There are many certifying organizations to choose from, but make sure that they are NCCA accredited. ACE and ACSM are the most widely recognized for group instructor certification. For yoga, you will need to do some more research. I do not know much about yoga certification. I do enjoy yoga though, but a good instructor program would need to be very indepth (as opposed to a weekend workshop or distance learning manual).